The present invention relates to a screw rotor machine for a working fluid and to the profiles of the rotors therefor. The invention is especially related to such a machine for selective compression and expansion of an elastic working fluid.
A screw rotor machine of the above type for an elastic working fluid comprises a casing with a working space provided with spaced apart low pressure and high pressure ports for communication with low pressure and high pressure channels, respectively, and generally composed of at least two intersecting bores with parallel axes, and a number of rotors intermeshing in pairs, disposed in said bores, each rotor having helical lands and intervening grooves with a warp angle of less than 360.degree.. A pair of communicating groove portions of intermeshing rotors form a chevron-shaped chamber having its base end disposed in a fixed plane transverse to the rotor axes, and adjacent to the high pressure port, whereas its apex moves axially as the rotors revolve to vary the volume of the chamber. One rotor of each pair is of female rotor type, i.e. a rotor having at least the major portions of its lands and grooves disposed inside the pitch circle of the rotor. The other rotor of the pair is of male rotor type, i.e. a rotor having at least the major portions of its lands and grooves disposed outside the pitch circle of the rotor. The lands of one rotor following the envelopes developed by the grooves of the other rotor to form a continuous sealing line between the rotors.
The efficiency of such machines depends to a large extent upon the profile of the rotors and desirably, the profile is such that each rotor groove is asymmetric about the radial line drawn from the centre of the rotor through the midmost point of the bottom of the groove and provided with a primary and a secondary flank. The primary flank is the trailing flank of the female rotor groove and the leading flank of the male rotor groove, respectively, when the machine acts as a compressor, and the reverse thereto when the machine acts as an expander, which means that the primary flank forms the peripherally outer wall of the leg of the chevron-shaped chamber composed of a female rotor groove and the peripherally inner wall of the leg of the chamber composed of a male rotor groove, respectively, and the secondary flank forms the other wall of the related leg of the chamber.
One such asymmetric rotor profile is disclosed in British Pat. No. 1,197,432 (based upon British patent application No. 34217/66), especially in FIGS. 6 and 7 thereof. In a plane perpendicular to the rotor axes the primary flank of each female rotor groove comprises a substantial concave portion following an epitrochoidal curve generally generated by a point near the radially outer end of the cooperating primary male rotor flank, a minor portion of the substantial portion extending out to the pitch circle and following a straight radial line, and a convex addendum portion following a circular arc having its centre adjacent to the pitch circle. The cooperating primary flank of a male rotor comprises correspondingly a substantial convex portion following an epitrochoidal curve generally generated by the innermost point of the minor portion of the primary flank of the female rotor groove, a minor convex portion extending to the pitch circle and following a curve being the envelope developed by the straight line defining the minor portion of the primary female rotor groove flank, and a concave dedendum portion generally following a circular arc and having its centre adjacent to the pitch circle. The secondary flank of the groove of the female rotor comprises a substantial concave portion out to the pitch circle and following a circular arc having its centre outside the pitch circle and having a tangent in its point of intersection with the pitch circle which tangent follows a radial line passing throught the centre of the rotor, and a convex addendum portion similar to that of the primary flank of the groove. The cooperating secondary flank of the male rotor comprises a convex substantial portion following the envelope developed by the circular arc defining the substantial portion of the secondary female rotor groove flank consequently having a radial tangent on the pitch circle and a concave dedendum portion similar to that of the primary flank of the land.
It has been found that the rotor profile so described is not ideal in all respects but impaired with drawbacks with regard to the portions of the male rotor flanks disposed adjacent to the pitch circle of the rotor. These drawbacks relate especially to the manufacture of the rotor and depend upon the angles of the flanks. Thus the angle between the two flanks of a male rotor groove at the pitch circle is so small that the angle between the axes of the rotor and of a cutter for its production is pratically fixed and necessitates substantially parallel edges of the cutting tool at the outer portion thereof. This means that it is practically impossible to produce the theoretical profile by a hob milling process.
Furthermore the variation along the flank of the angle between the tangent to the flank and a radius through the point of tangency as a function of the distance from the pitch circle is of generally hyperbolic type which means that it is substantially constant over the main portion of each flank but increases rapidly within the region thereof adjacent to the pitch circle. For this reason also, the milling cutter will get a fast variation of its angle at its outer end, i.e. a short radius of curvature, and consequently the cutting angels in the most important region of the rotor flanks will be unfavourable resulting in the necessity of relatively wide tolerances within this region. Further, the actual shape of the cutter induces a high wear thereof and thus a considerable amount of tool material has to be ground away during each resharpening operation. Consequently the number of required resharpenings is high and, since the number of possible resharpenings is limited, the cost of the tools is a real financial consideration which cannot be disregarded in the final cost of producing rotors. Still another drawback is that the radius of curvature of the flank decreases down to a zero value at the pitch circle. Such a curvature is very difficult to achieve which results in a poor and rough surface. However, even if a smooth surface is produced correctly, the short radius of curvature means that the surface will be exposed to very high surface stresses.
A modification of the rotor profile discussed above is disclosed in British Pat. No. 1,503,488 (based upon British patent application No. 10070/74). In this modified rotor profile, a section of the secondary flank of a female rotor groove disposed inside and adjacent to the pitch circle of the rotor follows in a plane perpendicular to the rotor axes a straight line forming a tangent to the circular arc defining the substantial portion of said second flank of the earlier discussed profile, and further forming an angle of 20.degree. with a radial line drawn from the centre of the rotor to the point of intersection between said flank section and the pitch circle of the rotor. The cooperating secondary flank of a male rotor is provided with a corresponding section disposed outside and adjacent to the pitch circle of this rotor and following the envelope developed by the straight line flank section of the female rotor. In this way the angle between the two flanks of a male rotor groove within the region adjacent to the pitch circle is increased up to a value allowing hob milling manufacture simultaneously as the radius of curvature of the secondary male land flank in its point of intersection with the pitch circle obtains a certain length which, however, is only about 60% of the product of the pitch radius and the sine function of said 20.degree. angle whereas the radius of curvature on the primary flank side still is zero. The variation along the flank of the angle between the tangent and the radius as a function of the distance from the pitch circle is still of hyperbolic type which means a rapid increase thereof adjacent to the pitch circle, even if this increase is not so accentuated as when the angle at the pitch circle goes down to a zero value. The drawbacks of the unmodified profile discussed above are thus in part eliminated, however, without resulting in ideal conditions. Furthermore, the lands of the female rotor will in this way be weakened which may cause problems during manufacture of the rotor as well as during operation of the machine owing to a certain bending of the lands.
The rotor profile shown in British Pat. No. 1,503,488 is further modified in relation to that shown in British Pat. No. 1,197,432 in that the dedendum portion of the primary flank of each male rotor adjacent to the pitch circle is provided with a section following a straight line directed radially towards the centre of the rotor and in that the addendum portion of the primary flank of each female rotor groove is provided with a corresponding section disposed adjacent to the pitch circle of the rotor and following the envelope developed by said flank section of the primary male rotor flank. Those sections of the primary flanks of the male and female rotors, respectively, are intended for an improvement of so called female rotor drive, i.e. the female rotor is connected to a prime mover and the male rotor is driven by direct flank contact between the rotors, which is intended especially for small compressors in order to increase the number or revolutions of the male rotor and thus the tip speed of the rotors without any need of a step-up gear. The location of those flank sections inside the pitch circle of the male rotor and outside the pitch circle of the female rotor, respectively, is intended to provide meshing conditions between those flank sections favourable for achieving a lubricating liquid film therebetween. However, the section of the primary flank of the female rotor groove will in its point of intersection with the pitch circle have a radial tangent, and a length of its radius of curvature having a zero value, similar to the conditions for the male rotor land flanks discussed above in relation to the unmodified profile. For this reason the section of the primary female rotor groove flank is impaired by drawbacks of about the same type as those discussed above with regard to the male rotor land flanks. Furthermore, the straight radial section of the primary male rotor land flank will further complicate the cutting of the rotor. Owing to those disadvantages a rotor profile as shown in British Pat. No. 1,503,488 is not suitable for practical use.
A further modification of the rotor profile shown in British Pat. No. 1,197,432 is shown in U.S. Pat. No. 4,053,263, where each flank of the male and female rotors adjacent to the related pitch circle is provided with a convex flank portion following a curve of involute type having a pressure angle of 20.degree.. This involute flank portion of the primary flank of each male rotor land extends from a point slightly inside the pitch circle to a point substantially at the outside diameter circle. The involute flank portion of each secondary flank of the male rotor extends from a point slightly inside the pitch circle to a point located a substantial distance outside the pitch circle. The involute flank portion of each flank of the female rotor extends between a point disposed slightly outside the base circle of the involute curve to a point slightly outside the pitch circle. In this way the angle between the two flanks of a male rotor groove is increased at the pitch circle simultaneously as the radii of curvature in the points of intersection between the flanks and the pitch circle gets a certain value, being the product of the pitch radius and the sine function of the pressure angle. However, the angle between the flanks of the male rotor decreases rapidly when moving inwardly from the pitch circle simultaneously as the variation of the angle between the tangent and the radius still is of generally hyperbolic type which means a rapid increase thereof adjacent to the pitch circle and in to the base circle of the involute. Furthermore the radii of curvature of the flanks also decrease rapidly within the radially innermost portions thereof. Eventhough this modified profile in spite of the relatively short radius of curvature of the male rotor flanks at the pitch circle may be acceptable for production of rotors where the directly contacting surfaces of the rotor flanks are disposed outside the pitch circle of the male rotor and inside the pitch circle of the female rotor, respectively, it does not allow more than a very small extension of those contacting surfaces beyond the related pitch circle. The modified rotor profile shown in U.S. Pat. No. 4,053,263 is consequently not suitable for production of rotors where the contacting surfaces of the male rotor lands extend inside the pitch circle which is especially essential for female rotor drive.
Still another modification of the rotor profile shown in British Pat. No. 1,197,432 is shown in British Pat. No. 1,358,505 where each female rotor groove flank inside of and adjacent to the pitch circle is provided with a convex flank section following a circular arc. The length of the radius of said arc is of the order of 20% to 40% of the centre distance of the rotors and the centre of said arc is disposed outside the pitch circle of the female rotor, which means that the cooperating flank section of the male rotor land in its point of intersection with the pitch circle has a tangent forming an angle of only about 5.degree. with a radial line drawn from the centre of the rotor through said point of intersection, and further that the radius of curvature of the flank section in said point is very small and amounts to only 60% to 70% of the product of the pitch radius and the sine function of said 5.degree. angle. The variation of the angle between the tangent and the radius is also in this modified profile of hyperbolic type and reaches a high value at the pitch circle. The advantages of this profile compared with that of British Pat. No. 1,197,432 are thus negligible.
A first main object of the present invention is to achieve a screw rotor machine of the type specified which may be manufactured more accurately and at a lower cost, simultaneously as the efficiency of the machine is improved, compared with earlier produced machines.
A second object is to achieve a screw rotor machine which may be adopted not only for male rotor drive but also for female rotor drive with at least the same efficiency and mechanical reliability.
A third object is to achieve a rotor profile where each female rotor land has such a shape that its peripheral width increases continuously from its radially outermost to its radially innermost end thereby increasing the stiffness thereof with regard to bending forces.
A fourth object is to achieve a continuous movement of the sealing point along each rotor flank from one end thereof to the other as the rotors revolve.